This application claims the priority of German application 197 17 537.6, filed Apr. 25, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention is based on a valve gear of an internal-combustion engine, and more particularly, to a valve gear having at least one charge cycle valve which is acted upon by a camshaft by way of a cam having at least two cam paths with different cam courses. A bucket tappet acts between the cam and the charge cycle valve and has at least two stroke transmitting devices which interact with different cam paths of the cam, of which one stroke transmitting element interacts with the valve stem of the charge cycle valve and the other stroke transmitting element interacts with a spring element whose spring effect on the stroke transmitting element is directed to the camshaft. The two stroke transmitting elements are coupled with one another by a displaceable coupling element in a first switching position and, in a second switching position, are movable independently of one another.
A valve gear is described, for example, in EP 0 515 520 B1 and has a tappet consisting of two concentric bucket elements. The interior bucket element of these elements rests with its one face against the valve stem of the charge cycle valve. The tappet interacts with the cam of a camshaft which has three partial cams with different cam plates. The two exterior cam plates have the same stroke course and act upon the exterior bucket element. The central partial cam has a stroke course which deviates from the former, has a lower stroke height and acts upon the interior bucket element.
The two concentric bucket elements in the known arrangement can be coupled with one another by the hydraulic action upon a coupling element or, in a second switching position of this coupling element, can be moved independently of one another. In the coupled switching position, the two bucket elements are connected with one another so that these follow the stroke course of the partial cams with a larger stroke. This movement is transmitted to the valve stem by way of the coupling element and the interior bucket element. In the second switching position of the coupling element, the two bucket elements can be moved independently of one another. In this switching position, the valve stem interacts with the central partial cam with the lower stroke.
The known exterior disk element follows the stroke movement of the exterior partial cams, in which case there is, however, no connection to the interior bucket element or to the valve stem. In the case of these tappets, however, the coupling element can be adjusted at any time out of its momentary switching position, as the result of a hydraulic action. Generally speaking, the displaceablility of the coupling element will exist only if all partial cams, interacting with the pertaining bucket element, are in their base circle phase because the coupling element is freely movable only in this switching position. The admission of pressure to the coupling element takes place independently thereof so that, under certain circumstances, the time for a complete adjustment of the coupling element from one switching position into another is not sufficient. This may undesirably stress the edges and result in high wear. In certain circumstances and in the event of an insufficient displacement, the coupling element can be pressed back by the forces acting out of the valve gear. As a result, after a partial stroke, the valve strikes back into the valve seat in an undamped manner which causes very disturbing noises and additional wear.
DE 44 05 189 A1 shows a valve gear of an internal-combustion engine which has a tappet for a charge cycle valve which can be switched off. The tappet has a coupling element for activating and deactivating the pertaining charge cycle valve. This coupling element is longitudinally slidable and has a bore into which the valve stem can dip in a switching position. In this switching position, a stroke movement of the tappet is possible which, however, is not transmitted to the valve stem. The displacement of the coupling element is possible only within defined cam paths. For this purpose, the coupling element interacts with a blocking device which consists of a resilient blocking tongue and an actuating pin. In defined positions, the resilient blocking tongue engages in the coupling element. The sensing pin takes measurements on a cam contour of the pertaining cam and transmits these measurements to the blocking element. Thereby, a relieving of the blocking element and therefore a displacing of the coupling element is possible only in defined cam path areas.
An object of the invention is to improve a valve gear of an internal-combustion engine such that undefined switching positions are avoided and the coupling element can always be changed securely from one of its end positions into the other end position. This simultaneously achieves the object of avoiding undesirable component stress by an insufficient carrying action.
According to the present invention, these objects have been achieved by providing that the coupling element has a locking contour which interacts with a locking element guided in the bucket tappet. The locking element interacts with the camshaft such that the coupling element as a function of the cam path can be locked in a first cam path range in a switching position and can be released in a second cam path range so that the coupling element can be displaced into the other switching position, and the locking element is acted upon by the spring element.
By constructing a locking contour on the coupling element which interacts with a locking element, which releases or blocks the coupling element as a function of the cam path, the coupling element can be displaced with assurance only within defined cam path areas. This operation assures that sufficient time always remains for the displacement of the coupling element during the base circle phase of the pertaining cam so that a secure switching-through of the coupling element will occur from one switching position into the other switching position. As the result of the direct engagement of the locking element into a locking contour constructed on the coupling element, component expenditures are also reduced which, on one hand, saves components and, on the other hand, saves installation space. The spring action upon the locking element further assures that the locking element is always loaded in the direction of the assigned cam contour. Also in the case of faulty positions of the coupling element, the locking element thus cannot jam but can be pressed over into a secure position against the effect of the spring element.
The cam-path-dependent blocking or releasing of the coupling element can advantageously be constructed such that a sensing element scans a cam contour of the camshaft and transmits it to the locking element. Thereby, in a first cam path area, the locking element locks the coupling element and releases it in a second cam path area. The locking element can, for example, scan an outside contour of the cam path or of the cam area. The locking of the coupling element can advantageously and without any additional components, take place by arranging the unlocking contour in the form of an elevation or indentation on the cam in the scanning area of the locking element. Thereby, the locking element is constructable in a particularly simple and low-cost manner as a longitudinally movable locking pin which engages directly in the locking contour on the coupling element.